Recently, a variety of copiers including analog and digital copiers have become widely used in business. The digital copier has become more popular than the analog copier because the digital copier can handle image data more easily.
Both analog and digital copiers generally employ a document reading apparatus. The document reading apparatus used in a conventional analog copier projects a document image directly onto a surface of a photoreceptor by an optical system to obtain copies of the document image.
On the other hand, the document reading apparatus used in a conventional digital copier forms the document image on a photoelectric conversion device such as a CCD (charge coupled device) by an optical system and converts the image into an electric signal. The document image is then written on a surface of a photoreceptor by an optical system based on the electric signal.
The document reading apparatus of the conventional digital copier exposes a light on a document with a lighting system, forms an image on a line sensor with light reflected from the document, and converts image information to signals. The line sensor performs photoelectric conversion.
Such a document reading apparatus generally uses an optical system which includes three mirrors and a reduction imaging lens. The optical system includes first and second running devices to read a whole area of the document by scanning the document. The first running device includes a first mirror and an illumination system. The second running device includes second and third mirrors. The first running device scans at a scanning speed twice as fast as the second running device.
FIG. 1 is an example of the document reading apparatus. The document reading apparatus includes a contact glass 1, first and second running devices 3 and 4, a line sensor 6 and a lighting system 7. In FIG. 1, a document 2 is placed on the contact glass 1. Lmax indicates a maximum reading range of the document 2.
The document provided on the contact glass 1 is illuminated by the lighting system 7 which is provided underneath the contact glass 1. A light from the document 2 is reflected at a first mirror 3a of the first running device 3, then is reflected at second and third mirrors 4a and 4b of the second running device 4. Further, the light from the document 2 is directed into a reduction imaging lens 5 to form an image on the line sensor 6.
When the document 2 is read in a longitudinal direction, the first running device 3 and the lighting system 7 move together at a speed V toward a position of the first running device shown by 3′. The position of the first running device shown by 3′ is the maximum reading range Lmax of the document 2. The second running device 4 moves at half the speed V/2 of the first running device 3 to a half moving distance of the first running device 3. The half moving distance of the first running device 3 can be referred by a position of the second running device shown by 4′. During the movements of the first and second running devices 3 and 4, the whole area of the document 2 is read.
FIGS. 2A and 2B are illustrations to explain reflection light path when the mirror has a curved surface. If the reflection mirror having a convex surface is used in the optical system of FIG. 1, the optical system has a negative power and diffuses a light beam as shown in FIG. 2A. If the reflection mirror having a concave surface is used in the optical system of FIG. 1, the optical system has a positive power and converges the light beam as shown in FIG. 2B.
If the optical system is used in the analog copier, two plane glasses each of which has a different type of mirror power may be employed so as to avoid being affected by magnification shift or to avoid resolution degradation due to length difference of light path for each image. As for the optical system used in the digital copier, width of imaging depth may be narrow in comparison to the conventional analog copier which performs same magnification imaging process because the magnification factor decreases from around 0.2 to around 0.1.
A total focal length of the mirrors and the lens may change due to a shift of positions between the mirrors and between the mirror and the lens. As a result, defocusing may occur and cause performance degradation.